Along with increased demand for communications, optical networks utilizing wavelength division multiplexing (WDM) techniques are widely deployed. WDM is a technology for multiplexing and transmitting a number of optical signals having different wavelengths.
A wavelength multiplexing transmission apparatus adopting the WDM technology has optical transceivers, known as transponders, for respective lines, and multiple optical signals are input and output via the optical transceivers. The wavelength multiplexing transmission apparatus transmits a wavelength-multiplexed optical signal generated by multiplexing optical signals input from optical transceivers to another apparatus, and splits a wavelength-multiplexed optical signal received from another apparatus into optical signals of given wavelengths and outputs them from the optical transceivers, for example.
The wavelength multiplexing transmission apparatus also includes an amplifier for amplifying outgoing and incoming wavelength-multiplexed optical signals. Thus, the reception level of a wavelength-multiplexed optical signal has to be within the dynamic range (the range in which light reception is possible) of the amplifier for avoidance of reception errors. Since the reception level varies with variations in transmission line loss caused such as by change in temperature and change in the laying condition of optical fiber (change in radius of curvature, for example), the wavelength multiplexing transmission apparatus controls the transmission level of wavelength-multiplexed optical signals so as to compensate for (or reduce) variations in transmission line loss.
In relation to transmission level control, Japanese Laid-open Patent Publication No. 5-030030 discloses a technique for detecting transmission line loss from the difference between the light output power of the transmitter of a bi-directional transmission apparatus and the received power of the receiver and adjusting the light output power, for example. Japanese Laid-open Patent Publication No. 9-116504 discloses a technique for branching an optical multiplexed signal received from an optical transmission terminal on a first transmission line, sending back the resulting signals to the optical transmission terminal on a second transmission line, and controlling light output power based on the result of measuring wavelength characteristics derived from the optical power of the signal light of each wavelength.
Japanese Laid-open Patent Publication No. 8-251103 discloses a technique for an optical transmitter to decide the state of connection of a connector that connects the optical transmitter with a transmission line based on whether a pilot signal is received from the transmission line or not and output optical signals to the transmission line only when the connector is connected.
A wavelength multiplexing transmission apparatus sends a wavelength-multiplexed optical signal to another apparatus on one of a pair of transmission lines and receives the value of reception level of the wavelength-multiplexed optical signal on the other transmission line from the other apparatus. The wavelength multiplexing transmission apparatus then calculates transmission line loss from the difference between the transmission level and reception level of the wavelength-multiplexed optical signal, and controls the transmission level so as to compensate for (or reduce) variations in transmission line loss.
The transmission level of a wavelength-multiplexed optical signal varies with the number of optical signals that are input from optical transceivers (the number of wavelengths corresponding to such optical signals, for example). When a change occurs in transmission level, it is recognized as a temporary variation in transmission line loss because the reception level used for calculation of transmission line loss has not reflected the change in transmission level due to transmission line delay. The wavelength multiplexing transmission apparatus therefore delays control of transmission level so that a calculated transmission line loss is not reflected into transmission level until the reception level value is updated.
This control method, however, is disadvantageous in that when transmission line loss significantly varies within a short period of time, the transmission level is not able to keep up with the variation due to the delay mentioned above, resulting in a reception error on the receiving-end apparatus.